Sources have said it is no longer so certain Van Gaal will see out the final year of his contract in 2016-17, and though much depends on performance in the remainder of this season, United have already begun pursuing Pep Guardiola as the Dutchman's successor.

With Manchester City believed to be leading the race for the Catalan, however, his old rival Mourinho would represent an ideal alternative for United in terms of profile -- if not managerial approach.

The prospect of Mourinho taking over at Old Trafford has long caused intense debate among the club hierarchy. The Portuguese is known to have wanted the job when Sir Alex Ferguson retired in 2013 and, while some key officials favoured hiring him then, others felt the controversy he brings and football style just didn't fit with United.

Jose Mourinho could take Louis van Gaal's place on the Manchester United bench.

At the same time, there is growing concern over results under Van Gaal -- especially the Champions League elimination at Wolfsburg -- as well as the lack of progress or sense of evolution. Sources say the United hierarchy are also hugely conscious of criticism that, in the two-plus years since Ferguson retired, they have missed out on three of the most sought-after managers in the world: Guardiola, Mourinho and Jurgen Klopp. Mourinho's availability in the summer could well offer the opportunity to end that criticism.

United executive vice chairman Ed Woodward also has a very strong relationship with the Portuguese's agent Jorge Mendes, and that would facilitate any move.

Sources have told ESPN FC that Mourinho has already received concrete offers for next season, and that Mendes has begun sounding out options.

Regarding his failure at Chelsea, those close to Mourinho say he grew hugely frustrated with the club's transfer structure, feeling that made it impossible to succeed at Stamford Bridge.